The invention related to a device for the contactless measurement of distance with ultrasound.
It is known to perform contactless measurements of distance with the aid of ultrasound transit-time measurements. In connection with device for warning of a collision in the case of motor vehicles, such as those known from the as yet unpublished patent application P 41 20 697, a short ultrasound signal is emitted by a sound transducer and, if an obstacle (reflector) is present, it is reflected by the latter and received by the sound transducer again. From the transit time, the distance between the sound transducer and the reflecting obstacle is calculated.
Since the level of the reflected signal depends not only on the level of the signal emitted but also on the distance between the ultrasound transducer and the reflector and on the quality of the reflector, problems arise in the evaluation of the echo signal.
One way of using the echo signal to generate a time marker is to compare the echo signal with a switching threshold in a threshold-value switch so that a signal is produced at the output of the threshold-value switch if the echo signal exceeds the threshold.
The level of the switching threshold must be fixed in such a way that, on the one hand, it is high enough to suppress unwanted extraneous echoes but, at the same time, it should also exceed the switching threshold in the case of a strongly absorbing reflector of the echo signal since no indication is otherwise possible.